Nonaqueous electrolyte secondary battery

ABSTRACT

An electric power generating element of a battery is covered with an electric conductor including a conductor member of a positive electrode electrically connected to a positive plate, a conductor member of negative electrode electrically connected to a negative plate, and a separating member, and an electric resistance per unit length of at least one of the conductor member of the positive electrode and the conductor member of the negative electrode is made to be smaller than that of the current collector electrically connected to the conductor member. For example, by making a thickness of the conductor member of the positive electrode to be larger than that of the current collector of the positive electrode, the electric resistance per unit length of the conductor member of the positive electrode can be made smaller than that of the current collector of the positive electrode. By adopting such a configuration, a large current occurring during internal short-circuiting can be rapidly distributed between the conductor members of the positive and negative electrodes, so that a remarkable effect can be achieved in preventing excessive heat generation of a battery.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a nonaqueous electrolyte secondary battery.

[0003] 2. Description of the Prior Art

[0004] Recently, concurrently going with the prevalence of such portable electric appliances as cellular telephones, notebook-type personal computers, or the like, there has been promoted the development of those batteries which can meet the demand for making these appliances to be miniaturized and to be provided with a high capacity. Among such batteries, the lithium ion secondary battery (hereinafter, sometimes simply referred to as “a battery”) is expected to meet such demand since it has a high operating voltage and a high energy density.

[0005] Incidentally, since a lithium ion secondary battery is required to have such a high energy density as described above, a variety of mechanisms are adopted for the purpose of securing the safety. In particular, there are available a separator having a function to shut down at high temperatures, a PTC device increasing in resistance with increasing temperatures, or the like.

[0006] However, in a larger-sized secondary battery having a larger capacity, the chemical energy quantity stored in the battery becomes larger, and hence it is more important to secure the safety.

SUMMARY OF THE INVENTION

[0007] The present invention has been developed in view of the above described circumstances, and an object of the present invention is to provide a nonaqueous electrolyte secondary battery in which the safety is secured even for such an extraordinary happening as a breakdown caused by external forces or the like.

[0008] When an internal short circuiting is caused by a breakdown due to external forces or the like, an extraordinarily large current is concentrated in the short circuited portion to generate the Joule heat and thereby elevate the temperature. In addition, sometimes the short-circuited portion comes across an instantaneous spark. Accompanying such a spark, there occur such reactions as a thermal decomposition of the positive active material, a reaction between either the positive or negative electrode and the electrolyte, and the like, and herewith the safety valve is caused to operate and sometimes fume is released.

[0009] In order to prevent the occurrence of such excessive heat generation, it is effective to provide the outermost layer of an electric power generating element with a configuration in which the element is covered with an electric conductor comprising the conductor member of positive electrode electrically connected to a positive electrode and the conductor member of negative electrode electrically connected to a negative electrode. With such a configuration, the initial stage of a short-circuiting can be allowed to occur within a portion of the conductor member in which portion no highly reactive active material is present, and thus there can be prevented the thermal decomposition of the active materials and the accompanying excessive heat generation.

[0010] In the case just mentioned above, by making the electric resistance of each conductor member be smaller than that of the current collector, the large electric current caused by a short-circuiting can be rapidly distributed over the electric conductor member, which has been found to be further remarkably effective in preventing the excessive heat generation of a battery. The present invention has been developed on the basis of such novel findings.

[0011] That is, the nonaqueous electrolyte secondary battery of the present invention, which is provided with an electric power generating element formed by laminating a positive electrode provided with a current collector of a positive electrode and a negative electrode provided with a current collector of a negative electrode with a separating member interposed therebetween, is characterized in that the outermost layer of the electric power generating element is covered with an electric conductor formed by laminating the conductor member of positive electrode electrically connected to the positive electrode and the conductor member of negative electrode electrically connected to the negative electrode with a separating member interposed therebetween, and the electric conductor member is made to be smaller in electric resistance than the current collector in at least one of the positive electrode part and the negative electrode part.

[0012] According to the present invention, the electric power generating element of a battery is covered with a conductor member of positive electrode electrically connected to a positive electrode and a conductor member of negative electrode electrically connected to a negative electrode, and the electric resistance of the electric conductor member is made to be smaller than that of the current collector. Thus, a large current accompanying a short-circuiting can be distributed in the electric conductor member, and hence a further remarkable effect can be achieved in preventing the excessive heat generation of a battery.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 is a perspective view of a battery of an embodiment of the present invention;

[0014]FIG. 2 is a perspective view of an electric power generating element and electric conductors wound together in an embodiment of the present invention;

[0015]FIG. 3 is an exploded perspective view of the electric power generating element and the electric conductors in an embodiment of the present invention;

[0016]FIG. 4 is a sectional view of the battery of an embodiment of the present invention; and

[0017]FIG. 5 is an enlarged sectional view of connected current collectors and electric conductors of the battery in an embodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

[0018] The nonaqueous electrolyte secondary battery of the present invention, which is provided with an electric power generating element formed by laminating a positive electrode provided with a current collector of a positive electrode and a negative electrode provided with a current collector of a negative electrode with a separating member interposed therebetween, is characterized in that the outermost layer of the electric power generating element is covered with the electric conductor formed by laminating the conductor member of positive electrode electrically connected to the positive electrode and the conductor member of negative electrode electrically connected to the negative electrode with a separating member interposed therebetween, and the electric conductor member is made to be smaller in electric resistance than the current collector in at least one of the positive electrode part and the negative electrode part. Incidentally, as for the electric resistance of a current collector and that of an electric conductor member per unit length of the present invention, those resistance values per unit length which are measured for a certain definite width are compared.

[0019] As for the material for the conductor members of positive and negative electrodes, there is no particular limitation, but there can be used any material exhibiting electric conductance; for example, such metals as aluminum, copper, titanium, iron, or the like are used in a foil form. In particular, in the positive electrode part, the use of aluminum foil is preferable in view of the high electrochemical stability thereof at higher voltages.

[0020] In either or both of the positive and negative electrode parts, the electric conductor member may be made to be smaller in electric resistance than the current collector. In order to make the electric conductor member be smaller in electric resistance that the current collector, for example, a material smaller in electric resistance than the current collector may be used for the electric conductor member. Alternatively, there may be used as the electric conductor member a member which is made of the same material as that for the current collector and larger in thickness than the current collector.

[0021] The use of a thicker electric conductor member is also effective in the point that the short-circuit current caused by a nail or the like driven into a battery can be rapidly distributed during the fairly long time required for the tip of the nail to reach the positive and negative electrodes. Furthermore, the use of the above thicker member is also effective in the point that the electric conductor member is not easily fused and electrically disconnected even when the temperature is increased in the portion connecting the current collector and the positive or negative electrode wherein the short-circuit current is concentrated.

[0022] According to the present invention, the electric power generating element of a battery is covered with the conductor member of positive electrode electrically connected to the positive electrode and the conductor member of negative electrode electrically connected to the negative electrode, and the electric conductor members are smaller in electric resistance than the current collectors. Consequently, a large electric current occurring during short-circuiting can be rapidly distributed, and hence a further remarkable effect is achieved in preventing the excessive heat generation of a battery.

[0023] Furthermore, the present invention is characterized in that the conductor member of positive electrode is electrically connected to the lead plate of positive electrode in which one end is connected to the current collector of positive electrode and the other end is connected to a positive electrode terminal, while the conductor member of negative electrode is electrically connected to the lead plate of negative electrode in which one end is connected to the current collector of negative electrode and the other end is connected to a negative electrode terminal.

[0024] According to the present invention, the electric power generating element of a battery is covered with a conductor member of positive electrode and a conductor member of negative electrode, and in addition, the conductor member of positive electrode is electrically connected to the positive electrode plate, and the conductor member of negative electrode is electrically connected to the negative electrode plate, so that a large current occurring in an extraordinary happening can be rapidly distributed in the conductor members, and hence a remarkable effect can be achieved in suppressing the temperature elevation in the battery. In this case, if at least either of the conductor member of positive electrode and the conductor member of negative electrode is made to be lower in electric resistance than the relevant current collector, a more remarkable effect can be achieved in suppressing the temperature elevation.

[0025] Particularly, the conductor member of the positive electrode is preferably thicker than the current collector of the positive electrode when aluminum foil is used for the current collector of the positive electrode and conductor member of the positive electrode. In this way, even when the heat liberation is caused by the short-circuit current flowing in a concentrated manner, the fusion and disconnection of the conductor member of the positive electrode can be prevented in the portion where it is connected to the positive electrode. In addition, by making the electric conductor member thicker, when a nail or the like is driven into a battery, a fairly long time is required for the tip of the nail to reach the internal positive or the negative electrode. Consequently, during this fairly long time, the short-circuit current can be distributed by the electric conductor member so that the short-circuit current generated between the positive and negative electrodes can be made smaller. Thus, the prevention of the excessive heat generation of a battery can be made further reliable.

[0026] With reference to FIG. 1 to 5, detailed description will be made of one embodiment in which a nonaqueous electrolyte secondary battery of the present invention has been implemented.

[0027]FIG. 1 shows a lithium ion secondary battery 1 in a completed form of the present embodiment (corresponding to a nonaqueous electrolyte secondary battery of the present invention; hereinafter, sometimes referred to as “battery 1”). The lithium ion secondary battery 1 comprises a casing 2, for example, formed with a metal in an elliptic cylindrical shape, and an electric power generating element 10 housed in the interior thereof, and an electric conductor 20 wound therewith.

[0028] The casing 2 is composed of a battery case 3 formed of a metal in a bottomed elliptic cylindrical container shape, and a metallic cover plate 4 formed in a nearly elliptical shape for sealing an opening of the battery case 3. The electric power generating element 10 is housed in the battery case 3, with two elliptical shape of insulator plates on the top and bottom thereof. The opening of the battery case 3 is sealed by welding the cover plate 4 thereto. A positive electrode terminal 5 and a negative electrode terminal 6, both made of a conductive material and of nearly a rod shape, are fixed around the center of the cover plate 4, in a manner passing therethrough. The terminals 5 and 6 pass through two through-holes 7 formed on the cover plate 4, and supported through insulating seals 8 by the cover plate 4 in an insulated state therefrom. Incidentally, a safety valve 9 is arranged on the cover plate 4 by covering a through-hole with a metallic thin film.

[0029] A mixed solution, for example, of ethylene carbonate (EC), diethyl carbonate (DEC), and dimethyl carbonate (DMC) in a ratio of 2:1:2, and an electrolyte with added lithium hexafluorophosphate in a concentration of 1 mol/l are charged into the interior of the casing 2.

[0030] The electric power generating element 10 has a configuration in which a belt-like positive plate 11 and a belt-like negative plate 14 are superposed on each other with a separator 17 (corresponding to a first separating member of the present invention) interposed therebetween, and are wound into a coreless shape (see FIGS. 2 and 3).

[0031] The positive plate 11 has a configuration in which a layer 13 of positive active material is formed on a current collector of positive electrode 12 made of a belt-like shape of aluminum foil of 20 μm in thickness. The positive active material layer 13 is formed almost all over each of both side surfaces of the current collector of positive electrode 12 excluding a thin margin on one lengthwise edge which edge side is to be the smaller end side when the positive plate is wound.

[0032] On the other hand, the negative plate 14 has a configuration in which a layer 16 of negative active material is formed on both the side surfaces of a current collector of negative electrode 15 made of a belt-like shape of copper foil of 16 μm in thickness. The negative active material layer 16 is formed almost all over both side surfaces of the current collector of negative electrode 15 excluding a thin margin on one lengthwise edge which edge side is to be the upper end side when the negative plate is wound.

[0033] These plates 11 and 14 are superposed on each other with separators 17 interposed therebetween made of polyethylene or polypropylene, and are wound to form the electric power generating element 10. In more detail, the superposition is made according to the order of the negative plate 14—the separator 17A—the positive plate 11—the separator 17B, and the winding is made with the negative plate 14 taking the inner side and with the outermost circumference taken by the exposed portion of the separator 17B. In this case, the negative plate 14 is superposed with a slight upward displacement, so that the upper edge margin provided with no negative active material layer 16 is made to protrude upward, while the positive plate 11 is superposed with a slight downward displacement, so that the smaller edge margin provided with no positive active material layer 13 is made to protrude downward.

[0034] The outer circumferential surface 10A of the electric power generating element 10 is covered with the electric conductor 20. The electric conductor 20 has a configuration in which a conductor member of positive electrode 21 made of an aluminum foil of 50 μm in thickness and a conductor member of negative electrode 22 made of a copper foil of 16 μm in thickness are laminated on each other with a separator 17C interposed therebetween, which separator corresponding to a second separating member of the present invention. The conductor member of positive electrode 21 is made to be larger in thickness than the current collector of positive electrode 12, so as to be smaller in electric resistance than the current collector of positive electrode 12. The electric conductor 20 is formed in a sheet shape as large as can cover in just proportion all over the outer circumferential surface 10A of the electric power generating element 10, and is wound around the outer circumferential surface 10A of the electric power generating element 10. Outside the electric conductor 20, a sheet of polyimide film 23 is wound for the purpose of insulating from the battery case 3.

[0035] A lead plate 31 of negative electrode is fixed to the top end face of the electric power generating element 10. The lead plate 31 of negative electrode is made of a strip of copper plate fabricated in a pleated form, and thus provided with a plurality of stripe-shaped terminal areas 32 comprising the pleats. To these terminal areas 32, the top end portion of the current collector of negative electrode 15 (where no negative active material layer 16 is formed) is inserted in a pinched form in such a way that the top end portion is sectioned into a plurality of groups each comprising apparently several sheets and each one group is allotted to one terminal area (see FIGS. 4 and 5). In addition, the conductor member of negative electrode 22 is inserted in a pinched form together with the current collector of negative electrode 15 above mentioned group into the terminal area 32 located in the outermost circumferential portion of the electric power generating element 10, so that the conductor member of negative electrode 22 and the current collector of negative electrode 15 are made to be electrically connected with each other. A piece of terminal plate 33 is made to project from an end portion of the lead plate 31 of negative electrode, and the piece of terminal plate 33 is connected to the negative electrode terminal 6.

[0036] A lead plate 34 of the positive electrode is fixed to the bottom end face of the electric power generating element 10. The lead plate 34 of the positive electrode is made of a strip of a nickel plate fabricated in a pleated form similarly to the case of the lead plate 31 of negative electrode, and thus provided with a plurality of stripe-shaped terminal areas 35 comprising the pleats. To these terminal areas 35, the bottom end portion of the current collector of positive electrode 12 (where no positive active material layer 13 is formed) and the conductor member of the positive electrode 21 are inserted in a pinched form similarly to the case of the negative electrode side, so that the conductor member of positive electrode 21 and the current collector of positive electrode 12 are electrically connected with each other. A belt shape of extension member 36 is arranged so as to abut on the outer circumferential surface of the polyimide film 23 from one end portion of the lead plate 34 of positive electrode and to reach a top portion of the battery, while the member 36 is also bent so as to abut on the top end face of the electric power generating element 10 and connected to the positive electrode terminal 5.

[0037] Now, description will be made below of the operations and performances of a lithium ion secondary battery constructed as described above.

[0038] When the lithium ion secondary battery 1 is used normally, the separators 17 insulate the positive plate 11 and the negative plate 14 from each other, and the conductor member of positive electrode 21 and the conductor member of negative electrode 22 from each other.

[0039] When such an edged piece of metal as a nail or the like is driven into the battery, and its tip reaches the interior portion of the battery 1, short-circuiting occurs between the positive and negative electrodes. Here, the outer circumferential surface 10A of the electric power generating element 10 is covered with the electric conductor 20, and hence the nail reaches first of all the electric conductor 20. Accordingly, short-circuiting occurs first of all between the conductor member of positive electrode 21 and the conductor member of negative electrode 22, and the short-circuit current flows. In this way, the initial short-circuiting is made to occur in the electric conductor 20 where the highly reactive active material layers 13 and 16 are not present, thereby suppressing the excessive heat generation of the battery 1.

[0040] Then, when the tip of a nail reaches the electric power generating element 10, the short-circuit current also starts to flow between the positive plate 11 and negative plate 14. It should be noted that the conductor member of positive electrode 21 is 50 μm thick to be thicker than the current collector of positive electrode 12 (20 μm thick), that is, the conductor member of positive electrode 21 is made to be smaller in electric resistance than the current collector of positive electrode 12. Accordingly, most of the large current during short circuiting can be rapidly distributed with the aid of the electric conductor 20, and the current flowing between the positive plate 11 and the negative plate 14 can be made small. Herewith, a further remarkable effect can be achieved in preventing the excessive heat generation in the battery.

[0041] Since the conductor member of positive electrode 21 is made to be thicker, when a nail or the like is driven into the battery, it takes a fairly long time for its tip reaches the electrode plates 11 and 14. During such a fairly long time, therefore, the short-circuit current can be distributed between the electric conductor members 21 and 22, and the short-circuit current generated between the plates 11 and 14 can be made smaller. Herewith, the prevention of the excessive heat generation in the battery 1 can be made further reliable.

[0042] In addition, in both electric conductor members 21 and 22, the portions around the portions connected to the lead plates 31 and 34 are possibly fused and electrically disconnected owing to the heat generated by the concentrated current therein during short-circuiting. In particular, aluminum used as the material for the conductor member of positive electrode 21 is larger in electric resistance, accordingly higher in the possibility of generating heat, and more liable to be fused and electrically disconnected than copper used as the material for the conductor member of negative electrode 22. Thus, by making the conductor member of positive electrode 21 be thicker, the fusion and electric disconnection can be prevented, the short-circuit current can be reliably distributed over the electric conductor 20, and the excessive heat generation of the battery 1 can be prevented.

[0043] According to the present embodiment, as described above, the electric power generating element 10 of the battery 1 is covered with the electric conductor 20 comprising the conductor member of positive electrode 21 electrically connected to the positive plate 11 and the conductor member of negative electrode 22 electrically connected to the negative plate 14. In addition, the conductor member of the positive electrode 21 is made to be larger in thickness than the current collector of the positive electrode 12, so that the conductor member of the positive electrode 21 is made to be smaller in the electric resistance per unit length than the current collector of positive electrode 12.

[0044] The electric resistance per unit length of the conductor member of the positive electrode 21 is preferably made to be equal to or smaller than ⅔ that of the current collector of positive electrode 12. Herewith, a large current during short-circuiting can be distributed between both conductor members 21 and 22, and accordingly a remarkable effect can be achieved in preventing the excessive heat generation of the battery 1. Furthermore, when the electric resistance of the conductor member of positive electrode 21 is equal to or smaller than ½ that of the current collector of positive electrode 12, the large current during short-circuiting can be more effectively distributed, and hence the temperature elevation in the short-circuited portion can be suppressed so that a more remarkable effect can be achieved in preventing the excessive heat generation.

[0045] Similarly to the above, the electric resistance per unit length of the conductor member of negative electrode 22 is preferably made to be equal to or smaller than ½ that of the current collector of negative electrode 15. Furthermore, for the purpose of rapid distribution of the large current during short-circuiting, the electric resistance per unit length of the conductor member of positive electrode 21 and that of the conductor member of negative electrode 22 are preferably equal to or smaller than ½ the electric resistance per unit length of the current collector of positive electrode 12 and that of the current collector of negative electrode 15, respectively.

[0046] The electric resistance per unit length of the conductor member of positive electrode 21 to be equal to or smaller than ⅔ or ½ that of the current collector of positive electrode 12 can be realized by making the thickness of the conductor member of positive electrode 21 be 1.5 or more times, or furthermore twice or more that of the current collector of positive electrode 12. Similarly, the electric resistance per unit length of the conductor member of negative electrode 22 to be equal to or smaller than ⅔ or ½ that of the current collector of negative electrode 15 can be realized by making the thickness of the conductor member of negative electrode 22 be 1.5 or more times, or furthermore twice or more that of the current collector of the negative electrode 15.

[0047] Furthermore, according to the present embodiment, the increased thickness of the conductor member of the positive electrode 21 can serve to suppress the fusion followed by electric disconnection in the portion connected to the lead plate 31, even when the heat is generated by the short-circuit current to flow in a concentrated manner. Furthermore, a fairly long time required for the tip of a nail or the like to reach the interior electrode plates 11 and 14 allows for the short-circuit current to be distributed between the conductor members 21 and 22, making the short-circuit current between the electrode plates 11 and 14 be smaller. Thus, the prevention of the excessive heat generation in the battery 1 can be made further reliable.

[0048] Incidentally, in the present embodiment, the electric conductor 20 is wound around the outer circumferential surface 10A of the electric power generating element 10, and a sheet of polyimide film 23 is wound around the outer surface of the electric conductor 20 thus wound. Alternatively, however, if the conductor member of the positive electrode 21 is the outer component of the electric conductor 20 and the material for the conductor member of the positive electrode 21 is the same as that for the casing 3, it is not necessary for the conductor member of positive electrode 21 to be insulated from the casing 3 by winding a sheet of polyimide film 23, but the member 21 and the casing 3 may be electrically connected.

[0049] With reference to Examples, more detailed description will be made below of the present invention.

EXAMPLE 1

[0050] 1. Fabrication of a Lithium Ion Secondary Battery

[0051] (1) Fabrication of a Negative Plate

[0052] Graphite as active material and poly (vinylidene fluoride) as binder were mixed in a composition ratio of 90:10 together with N-methylpyrrolidone (NMP) as the solvent for dissolving the binder, to prepare a negative composite paste. The paste was applied uniformly onto all over the two side surfaces of a current collector made of a sheet of copper foil of 16 μm in thickness, and the sheet of copper foil was dried and pressed, and cut out to fabricate a belt-like negative plate.

[0053] (2) Fabrication of a Positive Plate

[0054] LiNi_(0.55)Co_(0.15)Mn_(0.30)O₂ as active material, poly(vinylidene fluoride) as binder, and acetylene black as electric conductor agent were mixed in a composition ratio of 94:4:2 to prepare a positive composite paste. The paste was applied uniformly onto all over the two side surfaces of a current collector made of a sheet of aluminum foil of 20 μm in thickness, and a belt-like positive plate was fabricated in a manner similar to the case of the negative plate.

[0055] (3) Preparation of an Electrolyte

[0056] A nonaqueous solvent was prepared by mixing ethylene carbonate and diethyl carbonate in a volume ratio of 1:1. LiPF₆ as electrolyte was dissolved in the nonaqueous solvent in a concentration of 1.0 mol/dm³, to prepare a nonaqueous electrolyte.

[0057] (4) Fabrication of an Electric Conductor

[0058] A conductor member of positive electrode made of a sheet of aluminum foil of 50 μm in thickness and a conductor member of negative electrode made of a sheet of copper foil of 16 μm in thickness were laminated with a sheet of separator interposed therebetween to fabricate a belt-like electric conductor.

[0059] (5) Fabrication of a Battery

[0060] The positive plate, a separator made of polyethylene, the negative plate, and a separator made of polyethylene were laminated in this order, and were wound to fabricate an electric power generating element. The electric conductor was wound in a manner to come full circle around the outer circumferential surface of the electric power generating element. Furthermore, a sheet of polyimide film was wound around the outer circumferential surface of the electric conductor for the purpose of insulation. The current collector of positive electrode and the conductor member of positive electrode were electrically conductively connected by a lead plate of positive electrode. Similarly, the current collector of negative electrode and the conductor member of negative electrode were electrically conductively connected by a lead plate of negative electrode.

[0061] The electric power generating element was housed in a battery case made of aluminum. Then, the lead plate of positive electrode and the lead plate of negative electrode were respectively electrically connected to the positive electrode and negative electrode terminals which terminals were fixed to a cover plate. Subsequently, the cover plate was fixed to the battery case by means of laser welding. Finally, the electrolyte was charged into the battery case through a liquid feeding opening bored through the cover plate, and the opening was sealed by laser welding. The design capacity of the battery is 11.6 Ah, and as for the casing, the bottom surface is of the ellipse shape with the minor axis of 22 mm and the major axis of 60 mm, and the height is 100 mm.

[0062] A safety valve has been fixed to the cover plate of the casing, which valve is 8 mm in diameter, and the design opening pressure is 8 kgf/cm².

EXAMPLE 2

[0063] The battery of Example 2 was fabricated in the same manner as that in Example 1 except that a sheet of aluminum foil of 150 μm in thickness was used for the conductor member of positive electrode.

EXAMPLE 3

[0064] The battery of Example 3 was fabricated in the same manner as that in Example 1 except that a sheet of aluminum foil of 40 μm in thickness was used for the conductor member of positive electrode.

EXAMPLE 4

[0065] The battery of Example 4 was fabricated in the same manner as that in Example 1 except that a sheet of aluminum foil of 30 μm in thickness was used for the conductor member of positive electrode.

EXAMPLE 5

[0066] The battery of Example 5 was fabricated in the same manner as that in Example 1 except that a conductor member of negative electrode made of a sheet of copper foil of 16 μm in thickness and a conductor member of positive electrode made of a sheet of aluminum foil of 20 μm in thickness were laminated with a separator interposed therebetween to replace the conductor in Example 1.

COMPARATIVE EXAMPLE 1

[0067] The battery of Comparative Example 1 was fabricated in the same manner as that in Example 1 except that no electric conductor was used.

COMPARATIVE EXAMPLE 2

[0068] The battery of Comparative Example 2 was fabricated in the same manner as that in Example 1 except that a conductor member of negative electrode made of a sheet of copper foil of 16 μm in thickness and a separator were laminated to replace the conductor in Example 1.

COMPARATIVE EXAMPLE 3

[0069] The battery of Comparative Example 3 was fabricated in the same manner as that in Example 1 except that a conductor member of positive electrode made of a sheet of aluminum foil of 50μm in thickness and a separator were laminated to replace the conductor in Example 1.

[0070] 2. Nail Penetration Test

[0071] <Method of Testing>

[0072] The batteries of the above described group of Examples and that of Comparative Examples, three for each group, were subjected to the nail penetration test after the batteries were charged under the following charge condition of constant current and constant voltage.

[0073] (A) Condition of charge: 2.0 A×8 h (constant voltage: 4.1 V)

[0074] (B) Condition of charge: 2.0 A×8 h (constant voltage: 4.2 V)

[0075] The condition (A) of the above conditions of charge is an ordinary condition of charge, whereas the condition (B) is a condition of charge under which the battery in question is overcharged.

[0076] The nail penetration test was performed according to Japan Battery Industry Association Specification SBA G 1101 (Guideline for safety assessment of lithium ion secondary battery). An iron nail of 5 mm in diameter, 100 mm in length, and 30 degrees in point angle used for the test. Penetrating of a nail was made at a velocity of 24 mm/sec so as to penetrate the battery under test from the side surface of the battery case.

[0077] A K-type thermocouple was fixed to the central portion of the side surface of the battery case (about 10 mm away from the nail penetrating point) to measure the temperature variation during the nail penetration test.

[0078] <Results and Discussion>

[0079] Table 1 shows the results obtained by the nail penetration tests performed on the batteries of Examples and Comparative Examples. In TABLE 1, the symbol ◯ signifies that the safety valve was not opened, the symbol Δ that the safety valve was opened but no smoking (black smoke) occurred, and the symbol x that the excessive heat generation took place, the safety valve was opened, and smoking occurred therefrom. TABLE 1 Conductor Conductor Result of nail member of member of penetration test negative electrode positive electrode A B Example 1 Copper foil Aluminum foil ∘ ∘ (16 μm thick) (50 μm thick) Example 2 Same as above Same as above ∘ ∘ (16 μm thick) (150 μm thick) Example 3 Same as above Same as above ∘ ∘ (16 μm thick) (40 μm thick) Example 4 Same as above Same as above ∘ Δ (16 μm thick) (30 μm thick) Example 5 Same as above Same as above ∘ Δ (16 μm thick) (20 μm thick) Comparative None None x x Example 1 Comparative Copper foil None x x Example 2 (16 μm thick) Comparative None Aluminum foil x x Example 3 (50 μm thick)

[0080] The results of the nail penetration tests for the batteries charged under the condition of charge (A) are as follows. In any case of the batteries of Comparative Examples 1 to 3, rapid temperature elevation was observed immediately after nail penetration, the safety valve was opened, and fume was released from the battery, while the highest temperature on the side surface of the battery case was 400° C. or below. On the other hand, in any case of the batteries of Examples 1 to 5, the highest temperature on the side surface of the battery case was 110° C. or below, and the safety valve was not opened.

[0081] The results of the nail penetration tests for the batteries charged under the condition of charge (B) are as follows. In any case of the batteries of Comparative Examples 1 to 3, the safety valve was opened and fume was released from the battery, whereas in any case of the batteries of Examples 4 and 5, the highest temperature was 140° C. or below, the safety valve was opened, but fume was not released. In addition, in any case of the batteries of Examples 1 to 3, the highest temperature of the side surface of the battery case was 120° C. or below, and the safety valve was not opened.

[0082] From the above results, it has been found that, according to any battery of the present invention, the heat generation during the nail penetration test can be effectively suppressed and the safety can be secured.

[0083] In particular, as Examples 1 and 2 show, a remarkable effect was observed in preventing the excessive heat generation, when the thickness of the conductor member of positive electrode was made to be larger (1.5 or more times the thickness of the current collector of positive electrode, and more preferably twice or more).

[0084] In the above described Examples, the effect of the present invention was verified by making only the conductor members of positive electrode be thicker. If the conductor members of negative electrode (copper foil) is made to be thicker, the excessive heat generation in a battery can be more reliably performed, since the electric resistance of copper foil itself is so small that the short-circuit current can be more effectively distributed.

[0085] The technical scope of the present invention is not limited by the above described Examples, but those matters which will be described below, for example, are included in the technical scope of the present invention. In addition, the scope of the present invention encompasses the equivalent scope.

[0086] (1) In the present embodiment, the electric power generating element 10 is of the winding configuration. According to the present invention, the configuration of the electric power generating element is not limited to those in the present Examples, but it may be of the laminated configuration in which a plurality of sheets of the current collector of positive electrode and the current collector of negative electrode are laminated with separators each interposed between a conductor member of the positive electrode and a conductor member of the negative electrode. In such a case, belt-like conductors may be wound around the electric power generating element. In addition, a sheet-like conductor member of positive electrode and a sheet-like conductor member of negative electrode may be laminated on each of both outermost side faces of the electric power generating element with a separating member interposed therebewtreen.

[0087] (2) In the present embodiment, the electric conductor 20 capable of covering in just proportion the outer circumferential surface 10A of the electric power generating element 10 is wound in a manner to come full circle around the electric power generating element 10. According to the present invention, however, the number of conductor sheets wound is not limited to those in the present embodiments, but the conductor may be wound in two or more folds. In addition, a long length of belt-like conductor may be wound in two or more folds.

[0088] (3) In the present embodiment, only the conductor member 21 of the positive electrode is made to be thicker than the current collector of positive electrode 12. According to the present invention, however, the thicknesses of the conductor members are not limited to those in the present embodiment. Only the conductor member of the negative electrode may be made to be thicker than the current collector of the negative electrode. In addition, the conductor member of the positive electrode and the conductor member of the negative electrode may be made to be thicker than the current collector of the positive electrode and the current collector of negative electrode, respectively.

[0089] (4) In the present embodiment, the conductor member 21 of the positive electrode is used as one thick sheet. By using a conductor member 21 of the positive electrode comprising a plurality of superposed aluminum foil sheets, however, the total thickness of the conductor member 21 of the positive electrode maybe increased. This is also the case for the conductor member of positive electrode.

[0090] (5) In the present embodiment, the electric conductor 20 is wound around the outer circumferential surface 10A of the electric power generating element 10, and further a sheet of polyimide film 23 is wound around the outer circumferential surface of the electric conductor 20, while the conductor member 21 of the positive electrode is located outside the electric conductor 20. If the material for the conductor member 21 of the positive electrode is the same as that for the battery case 3, it is not necessary to wind a sheet of polyimide film 23 around all over the outer circumferential surface of the electric conductor 20 for the purpose of insulating the conductor member 21 of the positive electrode and the battery case 3 from each other, and the conductor member 21 of the positive electrode and the battery case 3 may be partially electrically connected to each other.

[0091] (6) In the present embodiment, the conductor member 21 of the positive electrode is arranged in the outer portion of the electric conductor 20, while the conductor member 22 of the negative electrode, in the inner portion. It is unnecessary to stick to this arrangement, so that the conductor member 22 of negative electrode may be arranged in the outer portion and the conductor member 21 of the positive electrode, in the inner portion, as far as the electric conductor 20 and the battery case 3 are insulated from each other.

[0092] (7) In the present embodiment, the conductor member 21 of the positive electrode in the electric conductor 20 is connected to the lead plate 34 of the positive electrode, and the conductor member 22 of the negative electrode, to the lead plate 31 of negative electrode. In addition to such a connection manner, the conductor member 21 of the positive electrode may be connected to a portion in the end of winding of the current collector 12 of the positive electrode, and the conductor member 22 of the negative electrode, to a portion in the end of winding of the current collector 15 of the negative electrode. 

1. A nonaqueous electrolyte secondary battery, comprising: a casing; a positive plate provided with a current collector of a positive electrode; a negative plate provided with a current collector of a negative electrode; a first separating member; a conductor member of the positive electrode electrically connected to said positive plate; and a conductor member of the negative electrode electrically connected to said negative plate, wherein an electric power generating element comprises said positive plate and said negative plate laminated on each other with said first separating member interposed therebetween, the outer circumferential surface of said electric power generating element is covered with the electric conductor comprising said conductor member of the positive electrode and said conductor member of the negative electrode laminated on each other with a second separating member interposed therebetween, and the electric power generating element is housed in said casing, and the electric resistance per unit length of at least one of said conductor member of positive electrode and said conductor member of the negative electrode is smaller than the electric resistance per unit length of said current collector electrically connected to the conductor member.
 2. The nonaqueous electrolyte secondary battery according to claim 1, wherein said current collector of the positive electrode and said conductor member of the positive electrode are made of aluminum foil, and the thickness of said conductor member of the positive electrode is larger than that of said current collector of positive electrode.
 3. The nonaqueous electrolyte secondary battery according to claim 1, wherein said current collector of the negative electrode and said conductor member of the negative electrode are made of copper foil, and the thickness of said conductor member of the negative electrode is larger than that of said current collector of the negative electrode.
 4. The nonaqueous electrolyte secondary battery according to claim 1, wherein said electric power generating element comprises said positive plate and said negative plate wound together with said first separating member interposed therebetween, and said casing is of an elliptic cylindrical shape.
 5. The nonaqueous electrolyte secondary battery according to claim 2, wherein said casing is made of aluminum or an aluminum alloy, said conductor member of positive electrode is arranged in the outer circumferential side of said electric conductor, and said conductor member of the positive electrode and said casing are electrically connected with each other.
 6. The nonaqueous electrolyte secondary battery according to claim 1, wherein the electric resistance per unit length of at least one of said conductor member of the positive electrode and said conductor member of the negative electrode is equal to or smaller than ½ that of said current collector electrically connected to the conductor member.
 7. The nonaqueous electrolyte secondary battery according to claim 2, wherein the thickness of said conductor member of the positive electrode is twice or more that of said current collector of positive electrode.
 8. The nonaqueous electrolyte secondary battery according to claim 3, wherein the thickness of said conductor member of the negative electrode is twice or more that of said current collector of negative electrode.
 9. A nonaqueous electrolyte secondary battery, comprising: a casing; a positive plate provided with a current collector of a positive electrode; a negative plate provided with a current collector of a negative electrode; a first separating member; a conductor member of the positive electrode electrically connected to said positive plate; a conductor member of the negative electrode electrically connected to said negative plate; a lead plate of positive electrode in which one end is electrically connected to said current collector of the positive electrode and the other end is connected to a positive electrode terminal; and a lead plate of the negative electrode in which one end is electrically connected to said current collector of the negative electrode and the other end is connected to a negative electrode terminal, wherein an electric power generating element comprises said positive plate and said negative plate laminated on each other with said first separating member interposed therebetween, the outer circumferential surface of said electric power generating element is covered with the electric conductor comprising said conductor member of the positive electrode and said conductor member of the negative electrode laminated on each other with a second separating member interposed therebetween, and the electric power generating element is housed in said casing, and said conductor member of the positive electrode is electrically connected to the lead plate of the positive electrode, and said conductor member of the negative electrode, to the lead plate of negative electrode.
 10. The nonaqueous electrolyte secondary battery according to claim 9, wherein the electric resistance per unit length of at least one of said conductor member of the positive electrode and said conductor member of the negative electrode is smaller than that of said current collector electrically connected to the conductor member.
 11. The nonaqueous electrolyte secondary battery according to claim 9, wherein said current collector of the positive electrode and said conductor member of the positive electrode are made of aluminum foil, and the thickness of said conductor member of the positive electrode is larger than that of said current collector of the positive electrode.
 12. The nonaqueous electrolyte secondary battery according to claim 9, wherein said current collector of the negative electrode and said conductor member of the negative electrode are made of copper foil, and the thickness of said conductor member of the negative electrode is larger than that of said current collector of the negative electrode.
 13. The nonaqueous electrolyte secondary battery according to claim 9, wherein said electric power generating element comprises said positive plate and said negative plate wound together with said first separating member interposed therebetween, and said casing is of an elliptic cylindrical shape.
 14. The nonaqueous electrolyte secondary battery according to claim 11, wherein said casing is made of aluminum or an aluminum alloy, said conductor member of the positive electrode is arranged in the outer circumferential side of said electric conductor, and said conductor member of the positive electrode and said casing are electrically connected with each other.
 15. The nonaqueous electrolyte secondary battery according to claim 9, wherein the electric resistance per unit length of at least one of said conductor member of the positive electrode and said conductor member of the negative electrode is equal to or smaller than ½ that of said current collector electrically connected to the conductor member.
 16. The nonaqueous electrolyte secondary battery according to claim 11, wherein the thickness of said conductor member of positive electrode is twice or more that of said current collector of positive electrode.
 17. The nonaqueous electrolyte secondary battery according to claim 12, where in the thickness of said conductor member of the negative electrode is twice or more that of said current collector of the negative electrode. 